Session expires when a modal window is opened

Hi,
I have Page1 having Button1.
Page2 having Button 2 and an ascx control UC1. UC1 has a Button 3. Page2 is a modal dialog( window.showModalDialog )
on Click of Button1 open Page2(modal dialog).
Page2 has UC1. UC1 has a button B3. On click of B3,instead of directly going to B3_click event, another post back happens. Thereafter opens login.aspx in another browser ( It is losing Session containing UserDetails as well as other session details).
now even If I continue entering Username and password, page2 reloads again in the new browser.
 
I am totally lost.
1) I do not know why UC1 button click makes another postback and thereafter opens up into another page losing all the sessions!!
I want to retain teh sessions till i ...